Various types of biometric systems are used more and more in order to provide an increased security for accessing an electronic device and at the same time keep the user convenience at an acceptable level. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important.
All capacitive fingerprint sensors provide an indicative measure of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially.
One of the problems associated with fingerprint sensors concerns the humidity level of the finger which is a fundamental problem for capacitive fingerprint sensors. For example, an increase in the humidity level may saturate the fingerprint image which presents verification difficulty and sometimes even rendering verification impossible. On the other hand, also low humidity of the finger may sometimes cause problems for fingerprint imaging due to e.g. coupling issues of the finger to the fingerprint sensor.
Thus, there is a need for improvements with regards to fingerprint verification based on fingerprint images acquired from fingers with high or low humidity levels.